This invention relates to lubricating oil compositions, e.g. automatic transmission fluids, heavy duty oils suitable for gasoline and diesel engines and cranckcase oils. These lubricating oil formulations conventionally contain several different types of additives that will supply the characteristics that are required in the formulations. Among these types of additives are included viscosity index improvers, antioxidants, corrosion inhibitors, detergents, dispersants, pour point depressants, antiwear agents, etc.
In the preparation of lubricating oil compositions, it is common practice to introduce the additives in the form of 10 to 80 mass %, e.g. 20 to 80 mass % active ingredient concentrates in hydrocarbon oil, e.g. mineral lubricating oil, or other suitable solvent. Usually these concentrates are subsequently diluted with 3 to 100, e.g. 5 to 40 parts by weight of lubricating oil, per part by weight of the concentrate to form finished lubricating oil compositions.
It is convenient to provide a so-called "additive package" comprising two or more of the above mentioned additives in a single concentrate in a hydrocarbon oil or other suitable solvent. However, a problem with preparing additive packages is that some additives tend to interact with each other. For example, dispersants having a high molecular weight or a high functionality ratio, for example, of 1.3 or higher, have been found to interact with other additives in additive packages, particularly overbased metal detergents. This interaction causes a viscosity increase upon blending, which may be followed by subsequent growth or increase of viscosity with time. In some instances, the interaction results in gelation. The viscosity increase can hamper pumping, blending and handling of the additive package. Although the additive package can be further diluted with more diluent oil to reduce viscosity in order to offset the effect of interaction, dilution reduces the economy of using an additive package by increasing shipping, storage and other handling costs.
U.S. Pat. No. 4,398,880 describes a process for improving the stability of oleaginous concentrates in the form of additive packages comprising ashless dispersants, particularly polyisobutylene containing dispersants, in combination with overbased metal detergents in which the additives are contacted in a lubricating oil basestock at a temperature of from 100.degree. C. to 160.degree. C. for 1 to 10 hours. The resultant heat-treated blend is then cooled to a temperature of 85.degree. C. or below and further mixed with copper antioxidant additives, zinc dihydrocarbyldithiophosphate antiwear additives and, optionally, other additives useful in lubricating oil compositions. The process enables the stability of the additive package to be improved to the extent that the tendency for phase separation is substantially reduced.
However, the molecular weight of the dispersant used in U.S. Pat. No. 4,398,880 is relatively low. The number average molecular weight of the polyisobutylene polymer used in the examples to make the dispersant is only 1725. The resulting dispersant number average molecular weight can be calculated to be approximately 3900 (e.g., 2 moles isobutylene polymer (MW=1725)+2 moles maleic anhydride (MW=98)+1 mole polyethyleneamine (MW=250)=2(1725)+2(98)+1(250).about.3900). The significant increase in viscosity due to the dispersant/detergent interaction, which will be described in more detail below, does not occur until the molecular weight of the polyisobutylene derivatized dispersant is much higher (i.e., approximately 7000).
There is a trend in the industry to go to higher molecular weight dispersants because they have improved dispersant properties to satisfy more rigorous performance requirements in the automobile industry. However, when higher molecular weight dispersants are used in concentrates, they interact with the colloidal overbased detergents to form a complex. This complex substantially increases the viscosity of the concentrate, which could result is blending difficulties unless the blending procedure is carefully designed.
Below is a simplified description of a concentrate containing an overbased detergent and an ashless dispersant. When an overbased detergent is added to an oil-based solvent, a colloidal structure forms containing hydrophilic groups and lipophilic groups, where the lipophilic groups extend out in the oil-based solvent. The ashless dispersant also contains hydrophilic groups and lipophilic groups. At sufficient concentrations, the dispersant interacts with the overbased detergent colloidal structure to form a dispersant/detergent complex where the hydrophilic groups of the overbased metal detergent colloidal structure interacts with the hydrophilic groups of the ashless dispersant.
Not wishing to be bound by any theory, it is believed that this dispersant/detergent complex causes an increase in viscosity because lipophilic groups of the ashless dispersant of one complex can interact with lipophilic groups of another complex. This results in an effective high molecular weight aggregate complex that increases the viscosity of the concentrate. The viscosity may rise uncontrollably to the extent that gels may form that are impossible to blend into a finished lubricating oil composition. The latter effect can evidence itself as the Weissenberg Effect. The Weissenberg Effect occurs when the viscosity of the concentrate significantly rises such that composition is seen to rise up the shaft of the mixing blades during blending.
It should be noted that the increase in viscosity would not occur if the concentration of the complex, or the molecular weight of the ashless dispersant in the concentrate is low. If the concentration of the complex is low (i.e., if the concentrate is dilute), there is sufficient space between the complexes such that the lipophilic groups of the dispersants will not interact. Likewise, if the molecular weight of the ashless dispersants is low, the lipophilic groups are too small to interact with each other. Thus, for example, a high molecular weight dispersant in a concentrate that is sufficiently dilute may not have a blending problem because there is sufficient space between the complexes such that an aggregate complex will not form. In contrast, a low molecular weight dispersant could have a blending problem in a highly concentrated composition because the space between the complexes is small. At typical additive package concentrations, the blending problems will not typically occur until the number average molecular weight of the dispersant is over about 7000 for polyisobutylene derivatized dispersants and over about 3000 for poly(alpha-olefin) derivatized dispersants.
However, the additive package concentrate should preferably contain a high molecular weight dispersant to satisfy performance requirements. In addition, the additive package should be highly concentrated to reduce shipping, storage and handling costs. Therefore, it is an objective of the present invention to provide a concentrated additive package composition that contains a higher molecular weight ashless dispersant than heretofore has been available due to viscosity considerations. It is also an object of the present invention to provide a process for preparing the additive package composition.